(What us with this uninteresting votes.... can't a man ask a question?)I think they are same. #597 has 'First production batch exclusive : AT field and AT field breaking hands' since it's limited release and #651 came out a year after without those as standard release it seems. Maybe they released due to popularity? As far as picture and size goes, they seems identical.
LOL Maybe because this could have been asked on page of the figure in question, but it WAS, so of course you'd probably try articles because they get more attention/answers. And their question is getting more attention this way, so it proves the point.
As for the difference I'm not sure, I saw your question posted on the item page, OP. I don't think these RAH have breakage/crumbling issues as far as I know (that was more 2008), but since I believe there is a kind of rubber on this figure coating the arms, rubber can and will crack over time (which is why I didn't buy these), rubber dries out and degrades. Think of an old rubber band and you'll get the idea.
First batch did have AT fields, but other than that I'm stumped why one would be worth more than the other.